Sequential Fe Reduction, Involving Two Different Fe+Intermediates, in the Conversion Reaction of Prussian Blue in Lithium-Ion Batteries

Ex situ X-ray absorption spectroscopy and in operando 57Fe-Mössbauer spectroscopy measurements are conducted to examine in detail the ongoing reaction mechanism of potassium Prussian blue (K-PB) within the narrow [1.6–0.005 V] voltage range, and so to determine whether this material truly undergoes a conversion reaction, as we proposed elsewhere. The generation of Fe0is confirmed by both techniques, finding that 40 to 58% of iron gets reduced to metallic iron at 5 mV. The elucidation of the mechanism by in situ 57Fe-Mössbauer spectroscopy further reveals a sequential process for the reduction (lithiation) of the two different iron species initially present in Prussian blue. Both high-spin Fe2+–N first and low-spin FeII–C next go through the unusual Fe1+/Iformal oxidation state during the reduction process, before forming surface Fe0nanoparticles (NPs) below 0.48 V. Upon charge, Fe0NPs preferentially oxidizes into Fe+–N. Interestingly, these surprising Fe+species play an important role in decreasing the overpotential during the charge (delithiation) process with respect to other conversion systems.